The present invention relates generally to current mirror circuitry, and, more particularly, to an improved current mirror having a cascode circuit forming a portion thereof permitting the current mirror to generate a constant-current output signal over an increased voltage range.
A communication system which transmits information between two locations includes, at a minimum, a transmitter and a receiver interconnected by a transmission channel. An information signal is transmitted by the transmitter upon the transmission channel, and the transmitted, information signal is received by the receiver to effectuate transmission of the information between the two locations.
A radio communication system comprises one type of communication system. In a radio communication system, the transmission channel is formed of a radio-frequency channel wherein the radio-frequency channel is defined by a range of frequencies of the electromagnetic frequency spectrum. In order for a transmitter of a radio communication system to transmit an information signal upon the radio-frequency channel, the information signal must be converted by the transmitter into a form suitable to allow transmission of the information signal upon the radio-frequency channel.
A process, referred to as modulation, converts the information signal into such form suitable to allow transmission thereof upon the radio-frequency channel. The transmitter contains circuitry to perform such modulation. In general, in a modulation process, an information signal is impressed (commonly referred to as "modulated") upon a radio-frequency electromagnetic wave. The radio-frequency, electromagnetic wave upon which the information signal is impressed (i.e., modulated) is commonly referred to as a "carrier signal", and the radio-frequency, electromagnetic wave, once modulated by the information signal, is referred to as a modulated, information signal, or, more simply, a modulated signal.
The modulated signal formed as a result of such a modulation process encompasses a range of frequencies centered at, or close to, the characteristic frequency of the carrier signal. The range of frequencies forming the bandwidth of the modulated signal is sometimes referred to as a modulation spectrum.
The modulated signal may be transmitted through free space upon the radio-frequency channel to transmit thereby the information signal between the transmitter and the receiver to effectuate the transmission of the information signal therebetween. Therefore, the transmitter and the receiver of a radio-frequency communication system need not to be positioned in close proximity to one another. As a result, radio communication systems are widely utilized to effectuate communication between a transmitter and a remotely-positioned receiver.
Many types of modulation techniques have been developed to modulate an information signal upon a carrier signal to form thereby the modulated signal. Such modulation techniques include, for example, amplitude modulation (AM), frequency modulation (FM), phase modulation (PM), frequency-shift keying modulation (FSK), phase-shift keying modulation (PSK), and continuous phase modulation (CPM).
The receiver of the radio communication system which receives the modulated signal transmitted upon the transmission channel by the transmitter contains circuitry to detect, or to recreate otherwise, the information signal modulated upon the carrier signal. Such circuitry, commonly referred to as demodulation circuitry, performs a process which, in essence, is the reverse of the modulation process utilized to form the modulated signal. The demodulation circuitry of the receiver, therefore, is of a construction corresponding to the type of modulation process utilized to form the modulated signal received by the receiver.
A plurality of modulated signals may be simultaneously transmitted as long as the simultaneously-transmitted, modulated signals do not overlap in frequency or otherwise do not interfere with one another. Modulated signals formed of carrier signals of dissimilar frequencies ensure that simultaneously-transmitted, modulated signals do not overlap in frequency.
Regulatory bodies have divided portions of the electromagnetic frequency spectrum into frequency bands, and have regulated transmission of modulated signals upon most of such defined frequency bands. The frequency bands have further been divided into channels, and such channels form the radio-frequency channels of a radio communication system. The defined radio-frequency channels are of bandwidths which ensure that a modulated signal centered thereupon does not interfere with a modulated signal simultaneously transmitted upon an adjacent channel. Regulation of transmission of modulated signals upon radio-frequency channels of the regulated frequency band minimizes interference between simultaneously-transmitted, modulated signals.
Both the modulation circuitry of the transmitter and the demodulation circuitry of the receiver contains mixing circuitry. Mixing circuitry of the modulation circuitry up-converts in frequency the information signal to form the modulated signal therefrom, and mixing circuitry of the demodulation circuitry down-converts in frequency a modulated signal to recreate the information signal therefrom.
Modulation circuitry converts the information signal, which is of a low (i.e., baseband) frequency, into a modulated signal of a transmission frequency having a modulation spectrum which must be of a bandwidth less than the bandwidth of a radio-frequency channel upon which the modulated signal is to be transmitted. The oscillating signals generated by the oscillators which are supplied to the mixing circuitry to up-convert the information signal in frequency must generate oscillating signals of precise frequencies to ensure that the modulation spectrum of the resultant, modulated signal is of such a bandwidth.
Oscillating signals of incorrect frequencies supplied to the various mixing circuitry can result in the resultant, modulated signal having a modulation spectrum beyond the range of frequencies defining the bandwidth of the radio-frequency channel upon which the modulated signal is to be transmitted.
To minimize such occurrences, transmitter circuitry frequently also includes phase locked loop circuitry which forms a feedback loop. Typically, the modulation circuitry of the transmitter includes mixer circuits to up-convert the information signal to a transmission frequency. Modulation circuitry having such a phase locked loop typically includes more than one oscillator in which one of the oscillators forms a reference oscillator to which other oscillators of the modulation circuitry are maintained in a frequency relationship. The phase locked loop compares the oscillating frequency of the reference oscillator with the oscillating frequency of the oscillating signals supplied to the mixing circuitry. Responsive to such comparison, the oscillating frequency of the oscillating signal applied to the mixing circuitry is altered. Such a feedback loop ensures that the oscillator which generates an oscillating signal supplied to mixer circuitry is of a desired frequency relative to an oscillating frequency of the reference oscillator of the transmitter.
Demodulation circuitry converts a modulated signal, of a transmission frequency, into a signal of a baseband frequency to permit recreation of the information signal. The demodulation circuitry of a receiver (analogous to the modulation circuitry of a transmitter) includes oscillators which generate oscillating signals which are supplied to mixing circuitry to down-convert in frequency a modulated signal received by the receiver, as above described, to recreate the information signal therefrom. Typically, the mixing circuitry of a receiver includes mixer circuits to down-convert a modulated signal to recreate the information signal therefrom. The oscillating signals generated by such oscillators must also be of precise frequencies to ensure proper recreation of the information signal subsequent to reception of a modulated signal.
Phase locked loop circuitry forming a feedback loop is also, therefore, frequently utilized to form a portion of the receiver circuitry. The mixing circuitry typically includes more than one oscillator in which one of the oscillators forms a reference oscillator to which other oscillators of the mixing circuitry are maintained in a frequency relationship. The phase locked loop compares the oscillating frequency of the reference oscillator with the oscillating frequency of the oscillating signals supplied to the mixing circuitry. Responsive to such comparison, the oscillating frequency of the oscillating signal applied to the mixing circuitry is altered.
Transceivers, such as two-way radios and radiotelephones are comprised of both a transmitter and a receiver forming transmitter portions and receiver portions of the transceiver, respectively. Transceivers typically include a single reference oscillator which generates an oscillating signal to which the oscillators of both the transmitter portions and the receiver portions of the transceiver are maintained in a desired frequency relationship. The transmitter portions and receiver portions, however, typically contain separate phase locked loop circuitry, as above described, to maintain the oscillating signals supplied to the mixing circuitry of the respective portions of the transceiver in the desired frequency relationships with the reference oscillator.
The oscillators which generate oscillating signals which are supplied to the mixing circuitry of the modulator of the transmitter and the demodulator of the receiver, respectively, are typically formed of voltage-controlled oscillators (VCOs) in which alterations of the frequencies of the oscillating signals generated therefrom may be varied by varying the level of a voltage signal applied thereto. Such alteration is sometimes referred to as "warping" of the oscillator. The phase locked loop circuitry, which forms the feedback control system, maintains the voltage controlled oscillator in the known frequency relationship with the reference oscillator.
The phase locked loop typically includes phase detector circuitry which is supplied with signals indicative of both the oscillating signal generated by the reference oscillator and signals indicative of the oscillating frequency of the voltage controlled oscillator. The phase detector compares the phases of the signals supplied thereto and generates a phase difference signal responsive to such comparisons. Such signal is typically supplied to a current mirror forming a portion of a charge pump. The charge pump generates the signals which are supplied to the voltage controlled oscillator (usually through a loop filter) to warp the frequency of the voltage control oscillator as needed.
The charge pump, of which the current mirror forms a portion, generates a current signal which may be of any of various voltages. The maximum voltage level at which a current mirror can generate an output signal is limited by the level of a source voltage less voltage drops across the current mirror circuitry. This is also the maximum voltage level of the output current signals generated by the charge pump.
Any increase in the range of voltages (and, more particularly, the maximum voltage level) of which the output current signal may be comprised reduces the sensitivity required of the voltage control oscillator. Such decrease in sensitivity required of the voltage controlled oscillator permits better control of the oscillating signal generated by the voltage controlled oscillator.
As the maximum voltage level of the output signal is related to the output impedance of the current mirror, an increase in such impedance level can improve the range of voltages over which the output signal may be generated. When the current mirror is comprised of a metal oxide semiconductor material, a high output impedance may be obtained by increasing the device length, i.e., by increasing length of the transistor. However, when the transistor length is increased, the width of the transistor must also be increased to prevent diminution of other transistor characteristics. An increase in the transistor width or length, however, increases the size of a gate electrode of the transistor, and the capacitance of the gate increases, and such increase results in slower rise and fall times of a current mirror formed therefrom.
Conventional cascode arrangements may be coupled to the current mirror. However, the maximum voltage level at which a current signal may be generated at the output of a current mirror when coupled thereto is reduced by the voltage drops across the transistors which comprise the conventional cascode arrangements.
What is needed, therefore, is an improved current mirror capable of generating a current signal over an increased range of voltages.